The present invention pertains to a process for preparing chemithermomechanical pulp and more particularly chemithermomechanical pulp that can be easily bleached using hydrogen peroxide in an alkaline medium.
To produce a pulp, the cellulosic substance, such as wood in the form of chips, is subjected to concurrent of separate mechanical, chemical or thermal operations.
The mechanical-type pulp of high-yield pulp is manufactured by mechanically grinding the lignocellulosic substance, for example, in a stone-type grinder or disc-type grinder or refiner.
Chemithermomechanical pulp, the preparation of which is referred to in this invention and called "CTMP" pulp in the text that follows, is produced by adding to the aforementioned mechanical process a nondestructive cooking operation of a lignocellulosic substance at a temperature equal to or greater than 100.degree. C. under saturated water steam pressure in the presence of sodium sulfite or sodium bisulfite or, more generally, a mixture of sulfur dioxide (SO.sub.2) and sodium hydroxide (NaOH), all referred to in the text that follows as "sulfite", unless otherwise specified.
The CTMP pulp is of a certain interest for the industry as it provides a valid compromise between mechanical pulp and chemical pump, per se.
For example, the CTMP pulp yield, pulp weight in dry state with respect to the weight of the starting substance in dry state, is generally greater than 85% and most often, at least equal to about 90%. This makes them very close to purely mechanically produced pump which is defined herein as pulp produced by the "nondestructive cooking of the substance".
However, the bleaching of the CTMP pulp, an operation required to obtain the required quality in this area of commercial products, poses a problem. Actually, this bleaching using hydrogen peroxide in an alkaline medium, for example, leads to inadequate results, probably due to the detrimental effects of heat to which the lignocellulosic substance is first subjected, particularly during the cooking operation defined above.
The aforementioned description is given in more detail, for example in the book by James F. CASEY, PUMP AND PAPER CHEMISTRY AND CHEMICAL TECHNOLOGY, 3rd ed. Vol. I, 1980, pp. 242-250, 654, 665.
For example, French Patent No. 1,150,451 recommends cooking at a temperature of up to 150.degree. C. using sulfite and hydrosulfite in an acidic medium. A possible bleaching of the ground pulp would be facilitated, but no particulars are given in this regard.
French Patent No. 1,389,308 notes that an improvement in the bleaching can be expected when treatment with an acid solution of the bisulfite ion HSO.sub.3.sup.-- is followed by a treatment, also in an acidic medium and without saturated water steam pressure, using sulfite in the presence of sodium borohydride which is preferred to hydrosulfite. The presence of sodium borohydride with sulfite in the first treatment is harmful and even leads to a less satisfactory result than that obtained when the sulfite is used alone in both treatments. Bleaching with H.sub.2 O.sub.2 of a pulp prepared according to the recommended mode leads to a degree of whiteness of well below 80%.
Finally, U.S. Pat. No. 3,981,765, which describes and completes the corresponding French Patent No. 2,186,984, states that when the intervention of sodium borohydride takes place in an alkaline medium, with or without sulfite, it is necessarily carried out in a medium having an initial pH of over 13, and should be followed, or preferably preceded by that of an acidic aqueous solution of SO.sub.2 or bisulfite. The procedure described is of practical interest only for hardwoods. Nothing is mentioned concerning the effect on the pulp by combining the alkaline treatment described with an alkaline treatment in the presence of hydrogen peroxide.
It is revealed in the known technique that, in a procedure for preparing a pulp prior to bleaching, which includes the use of sulfite and an agent such as hydrosulfite or borohydride, it is necessary to maintain a constant acidification phase.
Since the use of H.sub.2 O.sub.2 permits the bleaching of the pulp obtained using sulfite until it reaches a high level of whiteness, the industry is not interested in the combined action of sulfite with agents such as hydrosulfite or borohydride. As is known, the latter actually, amongst other things, results in a complication of the process, an additional consumption of SO.sub.2 or of bisulfite, and an increase in the pollution factor.